Fix for issue #18

app/src/main/java/org/zakariya/stickyheadersapp/ui/StressTestDemoActivity.java

@@ -1,6 +1,7 @@
 package org.zakariya.stickyheadersapp.ui;
 
 import android.os.Bundle;
+import android.os.Handler;
 
 import org.zakariya.stickyheaders.StickyHeaderLayoutManager;
 import org.zakariya.stickyheadersapp.adapters.SimpleDemoAdapter;
@@ -13,6 +14,17 @@ public class StressTestDemoActivity extends DemoActivity {
 	public void onCreate(Bundle savedInstanceState) {
 		super.onCreate(savedInstanceState);
 		recyclerView.setLayoutManager(new StickyHeaderLayoutManager());
-		recyclerView.setAdapter(new SimpleDemoAdapter(1000, 5, true, false, false, SHOW_ADAPTER_POSITIONS));
+
+		// this is a really crude emulation of a scenario where the layout manager
+		// has been assigned, but the adapter isn't created and assigned until data is
+		// made available (say, because of network latency).
+
+		Handler handler = new Handler();
+		handler.postDelayed(new Runnable() {
+			@Override
+			public void run() {
+				recyclerView.setAdapter(new SimpleDemoAdapter(1000, 5, true, false, false, SHOW_ADAPTER_POSITIONS));
+			}
+		}, 500);
 	}
 }

stickyheaders/src/main/java/org/zakariya/stickyheaders/StickyHeaderLayoutManager.java

@@ -94,29 +94,38 @@ public void setHeaderPositionChangedCallback(HeaderPositionChangedCallback heade
 	}
 
 	@Override
-	public void onAttachedToWindow(RecyclerView view) {
-		super.onAttachedToWindow(view);
+	public void onAdapterChanged(RecyclerView.Adapter oldAdapter, RecyclerView.Adapter newAdapter) {
+		super.onAdapterChanged(oldAdapter, newAdapter);
+
 		try {
-			adapter = (SectioningAdapter) view.getAdapter();
+			adapter = (SectioningAdapter) newAdapter;
 		} catch (ClassCastException e) {
 			throw new ClassCastException("StickyHeaderLayoutManager must be used with a RecyclerView where the adapter is a kind of SectioningAdapter");
 		}
-	}
 
-	@Override
-	public void onAdapterChanged(RecyclerView.Adapter oldAdapter, RecyclerView.Adapter newAdapter) {
 		removeAllViews();
 		headerViews.clear();
 		headerPositionsBySection.clear();
 	}
 
+	@Override
+	public void onAttachedToWindow(RecyclerView view) {
+		super.onAttachedToWindow(view);
+
+		try {
+			adapter = (SectioningAdapter) view.getAdapter();
+		} catch (ClassCastException e) {
+			throw new ClassCastException("StickyHeaderLayoutManager must be used with a RecyclerView where the adapter is a kind of SectioningAdapter");
+		}
+	}
+
 	@Override
 	public void onDetachedFromWindow(RecyclerView view, RecyclerView.Recycler recycler) {
 		super.onDetachedFromWindow(view, recycler);
 
 		// Update positions in case we need to save post-detach
 		updateFirstAdapterPosition();
-		adapter = null;
+		//adapter = null;
 	}
 
 	@Override
@@ -152,6 +161,10 @@ public void onRestoreInstanceState(Parcelable state) {
 	@Override
 	public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State state) {
 
+		if (adapter == null) {
+			return;
+		}
+
 		if (scrollTargetAdapterPosition >= 0) {
 			firstViewAdapterPosition = scrollTargetAdapterPosition;
 			firstViewTop = 0;
@@ -178,8 +191,9 @@ public void onLayoutChildren(RecyclerView.Recycler recycler, RecyclerView.State
 		int totalVendedHeight = 0;
 
 		// If we emptied the view with a notify, we may overshoot and fail to draw
-		if (firstViewAdapterPosition > state.getItemCount())
+		if (firstViewAdapterPosition > state.getItemCount()) {
 			firstViewAdapterPosition = 0;
+		}
 
 		// walk through adapter starting at firstViewAdapterPosition stacking each vended item
 		for (int adapterPosition = firstViewAdapterPosition; adapterPosition < state.getItemCount(); adapterPosition++) {